So what you can do is to evaluate $\displaystyle \displaystyle \begin{align*} X \end{align*}$ for each of your observed x values, and then do a least squares linear regression on the two sets $\displaystyle \displaystyle \begin{align*} X \end{align*}$ and $\displaystyle \displaystyle \begin{align*} y \end{align*}$. Your model will then be much more accurate, and then it can be written in terms of $\displaystyle \displaystyle \begin{align*} \log{(x)} \end{align*}$ again.